Method of preparing deoxidized steel

ABSTRACT

AN ADDITION AGENT IS PROVIDED FOR DEOXIDIZING AND RECARBURIZING VACUUM DEGASSED STEEL WHICH CONSISTS ESSENTIALLY OF CARBON, ALUMINUM AND/OR SILICON, MANGANESE AND IRON, AND A BINDER THEREFOR WHEN IN THE FORM OF BRIQUETTES. THE INVENTION ALSO PROVIDES A NOVEL METHOD OF DEOXIDIZING AND RECARBURIZING VACUUM DEGASSED STEEL WHICH UTILIZES THE ADDITION AGENT OF THE INVENTION. IN ONE VARIANT OF THE INVENTION, A MELT OF STEEL PREPARED BY A BASIC OXYGEN PROCESS IS VACUUM DEGASSED AND THEN DEOXIDIZED AND RECARBURIZED WITH THE ADDITION AGENT OF THE INVENTION TO PRODUCE MOLTEN KILLED STEEL HAVING AN ULTRA LOW OXYGEN CONTENT WHICH MAY BE CONTINUOUSLY CAST TO PRODUCE STEEL SHAPES WITH PROPERTIES SIMILAR TO RIMMED STEEL.

United States Patent() Int. Cl. C21c 7/06 US. C]. 75-49 16 Claims ABSTRACT OF THE DISCLOSURE An addition agent is provided for deoxidizing and recarburizing vacuum degassed steel which consists essentially of carbon, aluminum and/ or silicon, manganese and iron, and a binder therefor when in the form of briquettes. The invention also provides a novel method of deoxidizing and recarburizing vacuum degassed steel which utilizes the addition agent of the invention. In one variant of the invention, a melt of steel prepared by a basic oxygen process is vacuum degassed and then deoxidized and recarburized with the addition agent of the invention to produce molten killed steel having an ultra low oxygen content which may be continuously cast to produce steel shapes with properties similar to rimmed steel.

BACKGROUND OF THE INVENTION One recent important innovation in steel making is the casting of molten steel continuously into semifinished shapes such as blooms, slabs and billets. The successful continuous casting of shapes equivalent in section to conventional semifinished shapes eliminates the ingot and primary mill stages of conventional prior art rolled steel production, and thus affords important economic advantages.

Although the continuous casting of steel appears to be simple in principle, there are many difficulties inherent in the process. This is due in part to the'high melting point, high specific heat, and low thermal conductivity of steel, and the necessity for close control of variables such as the temperature of the molten steel in the tundish, which supplies molten steel to the water cooled mold. Additionally, it is necessary that the molten steel be killed sufficiently so that it will not effervesce in the mold to an extent such that blowholes form in the surface of the shapes as they are cast. Any tendency of themolten steel to elfervesce excessively and form blowholes in the thin skin that is initially formed around the molten steel interior of the shapes is very undesirable. This is true from the standpoint of weakening the skin and increasing the chance of escape of molten steel therefrom, as well as from the standpoint of surface imperfections. The initial thickness of the skin on the shape as it is withdrawn from the mold is dependent to some extent on the temperature of the molten metal in the tundish. Also, the initial temperature of the molten steel fed to the mold is of importance as the solidification rate must be predictable so as to form a skin of sufficient thickness and strength to support the casting. It is therefore apparent that an entirely successful continuous casting operation depends to a large extent upon providing deoxidized steel which is at a uniform proper casting temperature which does not vary greatly from melt to melt.

The melt of steel to be continuously cast may be prepared by any suitable prior art steel making practice. However, the basic oxygen process results in substantially lower costs and thus is often'preferred.

In accordance with one prior art basic oxygen process, the converter is charged with scrap, molten ferrous metal from a blast furnace, and other conventional charge materials necessary to produce a low carbon steel upon blowing with oxygen. In instances where the melt is to be cast into conventional ingots, the final temperature need not be higher than about 2900 F. and it is possible to stop theblow after the manganese, silicon, phosphorus and sulfur have been reduced to desirable levels. Under these conditions,the carbon remains at a level sufficiently high to impart strength to the steel even after vacuum degassing. However, in instances where the melt is to be cast continuously into semifinished shapes, the melt temperature should be about 50-75 F. higher, i.e., about 2950- 2975 F. and thus if the heats are overcharged with scrap it is necessary to continue the blow for a longer period of time to assure that the higher melt temperature is attained. This usually lowers the carbon content to about 0.04-0.06%. The oxygen content is about 500-700 parts per million, and it is necessary to deoxidize the steel before. continuous casting. Deoxidation may be effected by making aluminum and/or silicon additions, but in such instances the steel contains undesirably high aluminum, silicon, aluminum oxide and/or silicate contents for certain uses such as container stock. As a result, it is necessary to deoxidize the steel by vacuum degassing. The molten steel is degassed by subjecting it to a sharply reduced pressure, and preferably to a vacuum approaching 1-2 millimeters of mercury or less. Several suitable prior art processes are known, such as the Dortmund-Herder (D-H) process and the Ruhrstahl-Heraeus (R-H) process. In these processes, the molten steel is subjected to a vacuum in an evacuated vessel for a sufficient period of time to evolve the gases and for the gases to be removed by avacuum connection. The reaction of carbon with oxygen to produce carbon monoxide which is evolved is highly endothermic, and thus the initial temperature of the melt is reduced substantially in instances where it is desired to reduce the oxygen content to below parts per million. The carbon content is also reduced markedly, and usually to 0.03% or less. Thus insufficient carbon remains in the steel to provide strength for certain end uses such as container stocks.

In view of the foregoing, the preparation of blackplate strip or sheet for container stock presents a series of problems which have not been adequately solved by the prior art'in instances where it is desired to combine a basic oxygen steelmaking process with vacuum degassing and continuous casting.

SUMMARY OF THE INVENTION- The present invention provides a novel exothermic addition agent which is especially useful for deoxidizing and recarburizing vacuum degassed steel. The addition agent consists essentially of carbon, aluminum and/or silicon, and iron. A binder may be present when the ingredients are in particulate form initially and it is desired to form briquettes therefrom.

The invention also provides a novel method of deoxidizing and recarburizing vacuum degassed steel which utilizes the above described addition agent of the invention. Inasmuch as the aluminum and/or silicon contents of the addition agent react exothermically with oxygen contained in the molten steel, this offsets the endothermic reactionbetween carbon and oxygen and aids in maintaining the desired temperature of the melt for continuous casting.

In one preferred variant of the invention, steel prepared by a basic oxygen process may be vacuum degassed, and then further deoxidized and recarburized with the addition agent of the invention to produce a molten killed steel having an ultra low oxygen content. The molten steel remains at a sufficiently high temperature for continuous casting, and it is possible to produce semifinished steel shapes therefrom having properties similar to rimmed or capped steel. The carbon content is sufficiently high to impart strength and allow the finished steel to be used where rimmed or capped steel has been used heretofore.

DETAILED DESCRIPTION OF THE INVENTION INCLUDING PREFERRED VARIANTS THEREOF The addition agent of the invention consists essentially of (1) carbon, (2) aluminum and/or silicon, (3) manganese and (4) iron. Each ingredient has an important function and contributes to the unique properties of the overall composition. For instance, the carbon recarburizes the vacuum degassed steel, increases strength, and provides additional carbon for reaction with oxygen in the melt to produce carbon monoxide and carbon dioxide which are evolved. The aluminum and/or silicon react exothermically with oxygen contained in the molten steel and reduce the reaction rate between the carbon and oxygen. The carbon alone reacts violently with the oxygen content of the melt, and this causes metal splashing and spitting into the vacuum duct of the degassing apparatus which is undesirable. The ductwork of the degassing apparatus is refractory lined in the vicinity of the vacuum chamber, and steel deposited on the refractory linings will later be oxidized by the preheater or gas burner. The iron oxide thus produced will attack the refractory linings and cause excessive wear and eventually premature failure. The aluminum and/or silicon also deoxidize the molten steel, and the exothermic reaction thereof with oxygen offsets at least to some extent the heat loss due to the endothermic reaction of the carbon with oxygen. The iron and manganese are economic specific gravity increasing agents or sinkers, and the manganese also insures an increase in the manganese content of the steel which is desirable.

The ratio by weight of carbon to the exothermic substance, i.e., aluminum, silicon, and mixtures or alloys thereof, should be between about 1:4 and 3:2, and is preferably about 1:1. The manganese, iron and mixtures or alloys thereof may be present in a combined amount by weight of at least 25%, and preferably at least 50% of the total weight of carbon and the exothermic substance. However, for best results the weight of manganese and/or iron should not exceed the combined weight of carbon and the exothermic substance.

One specific additive composition which is especially useful contains 30-40 parts by weight of carbon, 30-40 parts by weight of aluminum, and 20-40 parts by weight of combined manganese and/ or iron. Still another specific composition which is especially useful contains 30-40 parts by weight of carbon, 30-40 parts by weight of silicon, and 20-40 parts by weight of combined manganese and/or iron.

The sources of carbon need not differ from those used in the prior art for recarburizing molten steel. For example, the carbon source may be graphite, carbon black, petroleum coke, coke prepared from coal, or other suitable carbonaceous materials. The particle size of the carbon is not critical and may vary from the microscopic particles in carbon black to granules of graphite or coke of a size suitable for briquetting.

In instances where the exothermic substance is aluminum, it may be in the form of, for example, aluminum shot, aluminum turnings, aluminum powder or granules, chopped aluminum wire, shredded aluminum sheet, or suitable aluminum alloys and aluminum containing compositions which do not contain deleterious ingredients. When silicon is the exothermic substance, it may be in the form of, for example, elemental silicon, ferrosilicon, silicomanganese, and other suitable alloys or silicon-containing substances which do not include deleterious ingredients. The silicon-containing material may be in a particulate form such as powder, pellets, granules or fines of a size suitable for briquetting.

The manganese and/ or iron may be present as metallic manganese, metallic iron, ferromanganese, or other suitable manganese and/ or iron-containing substances which do not include deleterious ingredients. The manganese and/ or iron-containing material may be, for example, in the form of shot, pellets, granules or fines of a size suitable for briquetting. The manganese and/ or iron are preferably present in an amount to increase the specific gravity of the mixture sufiiciently to assure that the additive will be immersed in or sink into the ferrous metal melt to be treated.

When a binder is present and the ingredients are in particulate form, the addition agent may be formed into desired shapes or compacts. In most instances, the binder should be present in an amount of about 1-10 parts by weight for each parts by weight of the remaining ingredients, and preferably in an amount of about l-S parts by Weight. The specific nature of the binder is not of importance so long as it is effective and does not contain substances which would be deleterious, but preferably the binder is a substance which will decompose at the temperature of the melt and leave little or no residue. The binder may be in accordance with prior art practice, it may include one or more substances such as molasses or other sugar containing substances, starch and starch derivatives, bituminous materials such as coal tar, petroleum asphalt, pitches, and their component fractions including resins and asphaltenes, proteinaceous derivatives or adhesives such as casein and glue, synthetic thermoplastic and thermosetting resins such as phenol-formaldehyde resins, and inorganic materials such as alkali metal silicates and other water soluble silicates. It is understood that still other suitable prior art binders for metallurgical additives may be employed when desired.

The binder may be mixed with the remaining ingredients in particulate form, and briquettes or shapes may be prepared by applying pressure thereto, balls or generally spherical shapes may be prepared by molding or com pacting the mixture, the mixture may be extruded in the form of elongated or rod-like shapes which are cut into desired lengths, or other suitable methods may be used. Regardless of the method of manufacture or the configuration of the shapes, they are referred to herein as being briquettes for purposes of simplifying the discussion. The briquettes may be of any suitable size, but are preferably not more than about 3 inches in one dimension. The briquettes may be, for example, about 0.25-3 inches, but better results are usually obtained with briquettes having a size of about 0.5-2 inches. Briquettes having a generally spherical configuration and a diameter of about 1-3 inches usually give the best results.

In instances where a binder is not employed and the ingredients are in particulate form, the mixture may be partially fused or sintered to cause the particles to adhere together upon cooling. In instances where the agglomerated product thus produced contains particles which are too large, they may be reduced in size by crushing to the size range noted above for the briquettes. It is also possible to pack the ingredients into ferrous metal containers without a binder being present, and then apply pressure to the filled containers to thereby compact the same and increase the specific gravity. The compacted filled containers may be added to ferrous metal melts in the same manner as the briquettes referred to herein.

When practicing the method of the invention, the addition agent is added to a melt of steel which is subjected to a reduced pressure to produce degassed steel in an amount to further deoxidize and recarburize the steel to a desired extent. For instance, one or more portions of the addition agent may be added to the steel while it is being vacuum degassed and/ or after it has been vacuum degassed to a desired oxygen content. Preferably, at least some degassing is performed after the addition agent has been added to aid in removing any additional carbon monoxide that is evolved.

The amount of the addition agent to be used may vary over wide ranges, but preferably it is added in an amount to provide no more than 0.05 by weight, and preferably less than 0.02% by weight, of residual aluminum and/or silicon in the treated steel. 1n instances where the steel melt to be treated is prepared by a basic oxygen process, then the addition agent may be added at a rate to provide the carbon and the exothermic substance in a combined amount of about 0.5-4 pounds per ton of steel, and preferably in an amount of about 2.0-2.5 pounds per ton of steel. The best results are often obtained with about 2.25 pounds of the carbon and exothermic substance per ton of steel.

The combination of vacuum degassing and the deoxidizing effect of the exothermic substance is suflicient to assure that the deoxidized and recarburized steel will not effervesce in the mold sufiiciently to produce a rimming action or surface blowholes when the steel is continuously cast. The low carbon steels produced by a basic oxygen process and which are preferred in practicing the present invention contain not more than 0.15% and often less than 0.10% carbon, and in most instances about 0.06-0.08% carbon. The manganese content is less than 1.0% and usually about 0.250.75%, the silicon content is less than 0.05% and usually below 0.02%, the phosphorus content is less than 0.05% and usually below 0.02%, the sulfur content is less than 0.05 and usually below 0.03%, and the remainder is iron and incidental impurities. Trace amounts of tramp elements such as copper, tin, lead, zinc and the like may be present, but their concentrations are very low. The oxygen content of the steel is below 1000 parts per million (p.p.m.), and is usually about 500-700 p.p.m.

When a steel of the above type is subjected to vacuum degassing by the D-H or R-H process the carbon content is lowered to about 0.02-0.04% and the oxygen content to approximately 200 p.p.m. After adding the addition agent of the invention, the carbon content may be raised to its initial value of about 0.06-0.08%, and the oxygen content is lowered to below 100 p.p.m. Thus in one preferred variant, the addition agent is added in an amount to recarburize low carbon vacuum degassed steel produced by a basic oxygen process to provide a final carbon content which is approximately the same as that which existed prior to degassing. The oxygen content of the degassed and recarburized steel thus produced is less than 100 p.p.m.

The temperature of the recarburized killed steel is not reduced markedly and it may be continuously cast without difiiculty in accordance with prior art practices to produce semifinished steel shapes such as blooms, slabs and billets. The semifinished shapes may be finished into final products in accordance with prior art practices. For instance, slabs may be rolled to produce blackplate sheet or strip for container stock as the carbon content in the recarburized steel is sufficiently high for this purpose. The steel produced by the continuous casting process has the general properties of rimmed or capped steel and the aluminum and/or silicon contents are sufficiently low so that the rigid container stock specifications may be met with respect to these substances.

Basic oxygen steelmaking processes and apparatus, vacuum degassing processes and apparatus, and continuous casting processes and apparatus for use in preparing melts to be vacuum degassed and continuously cast in accordance with the invention are disclosed in numerous patents and literature references, and in the text The Making, Shaping and Treating of Steel, 8th edition, edited by Harold E. McGannon (1964), the disclosures of which are incorporated herein by reference. Pages 453-456, 552- 556, and 664-666 of The Making, Shaping and Treating of Steel are especially pertinent.

The invention is further illustrated by the following specific examples.

EXAMPLE I A melt of steel is prepared by a basic oxygen process following prior art steelmaking practices, and is transferred from the converter to a ladle for degassing. The ladle contains 300 tons of the molten steel. Prior to degassing, the steel has a temperature of 2950 F. and contains 0.06% carbon, 0.35% manganese, less than 0.01% silicon, less than 0.01 phosphorus, less than 0.025% sulfur, 600- 700 parts per million of oxygen, and the remainder iron and trace amounts of incidental impurities.

The steel is degassed employing conventional R-H degassing apparatus and techniques until the oxygen content is reduced to 200 parts per million. At this time, the steel contains only 0.02% carbon.

One inch briquettes prepared from a particulate mixture containing 30 parts 'by weight of graphite, 30 parts by weight of aluminum, 35 parts by weight of ferromanganese, and 5 parts by weight of molasses as a binder are added in an amount of 1300 pounds. The degassing is allowed to proceed after the addition until a constant vacuum is reached thereby indicating that the steel is degassed.

After degassing, deoxidizing and recarburizing the melt by the above procedure, the carbon content is 0.06% and the oxygen content is less than parts per million. The temperature of the melt is 2910 F. and the molten steel is continuously cast following prior art practices to produce slabs. The slabs are then hot and cold rolled to produce blackplate which meets all specifications for container stock, including tinplate specifications, as the silicon content is less than 0.02%.

EXAMPLE II The general procedure of Example I is repeated except as noted below.

The additive is in the form of two inch spheres or balls containing 25 parts by Weight of carbon, 25 parts by weight of silicon (as ferrosilicon), 45 parts by weight of manganese and iron (as ferromanganese and ferrosilicon) and 5 parts by weight of molasses as a binder. The additive is added in an amount of 1600 pounds, rather than 1300 pounds as in Example I, but otherwise the degassing, deoxidizing and recarburizing procedure is the same.

The blackplate product meets all specifications for galvanized sheet stock and the results in general are substantially the same as in Example I, except for a slightly higher silicon content. The steel could also be hot rolled and/or cold rolled to produce other products in sheet or strip form, or structural shapes, plate, etc.

EXAMPLE III A melt of steel is prepared and transferred to a ladle following the general procedure of Example I. This specific melt contains 0.06% carbon and 400-500 parts per million of oxygen, but otherwise the composition is the same as in 'Example I. The melt is degassed using the same :R-H apparatus that was used in Example I, but the procedure is changed as noted below. The steel is degassed for 10 minutes, during which time the vacuum stabilizes, and then 1200 pounds of the briquettes of Example I are added at the rate of 200 pounds per minute to the 300 tons of steel in the ladle. The vacuum cycle is continued during the addition and for approximately 5 minutes after the addition is completed.

The degassed, deoxidized and recarburized molten steel contains approximately. 0.06% carbon, and substantially les than 100 parts per million of oxygen. The molten steel may be continuously cast to produce slabs and the slabs rolled to produce excellent blackplate for container stock as in Example I.

I claim:

1. A method of preparing deoxidized steel comprising preparing a heat of unkilled steel containing not more than 0.15 carbon, less than 1.0% manganese, less than 0.1% silicon, less than 0.05 phosphorus, less than 0.05% sulfur, less than 1000 parts per million of oxygen and the. remainder iron and incidental impurities, vacuum degassing said heat of steel to produce molten partially deoxidized vacuum degassed steel having substantially lower oxygen and carbon contents than were present initially and adding an addition agent in the form of a plurality of discrete shapes to said molten vacuum degassed steel to further deoxide the steel, said plurality of discrete shapes of the addition agent being added to said molten vacuum degassed steel in an amount to complete the deoxidation thereof and to produce molten vacuum degassed and recarburized steel, said plurality of discrete shapes of the addition agent each having a composition consisting essentially of (1) carbon, (2) at least one substance which reacts exothermically with oxygen contained in molten steel selected from the group consisting of aluminum and silicon, (3) manganese, and (4) iron, the said plurality of discrete shapes of the addition agent containing carbon in an amount of about 30-40 parts by weight, the exothermic substance in an amount of about 30-40 parts by weight, and the manganese and iron in a combined amount of about 20-40 parts by weight.

2. The method of claim 1 wherein the said exothermic substance of the addition agent is aluminum.

3. The method of claim 1 wherein the said exothermic substance of the addition agent is silicon.

4. The method of claim 1 wherein the said carbon, exothermic substance, manganese and iron of the addition agent are in particulate form initially, a binder therefor is also present, and the addition agent is in the form of briquettes.

5. The method of claim 1 wherein the said carbon, aluminum, manganese and iron of the addition agent are in particulate form initially, a binder therefor is also present in an amount of about 1-10 parts by weight, and the addition agent is in the form of spheres having a diameter of about 1-3 inches.

6. The method of claim 1 wherein the addition agent is added in an amount to recarburize the steel to approximately the carbon content that existed prior to degassing.

7. The method of claim 1 wherein said heat of steel is produced by a basic oxygen process, and the addition agent is added in an amount to provide the said carbon and exothermic substance in a combined amount of about 0.5-4 pounds per ton of steel.

8. The method of claim 1 wherein said heat of steel is produced by a basic oxygen process, and the addition agent is added .in an amount to provide the said carbon and exothermic substance in a combined amount of about 2.0-2.5 pounds per ton of steel.

9. The method of claim 1 wherein the addition agent is in the formof spheres having a size of about 1-3 inches, the addition agent is added in an amount to recarburize the steel to approximately the carbon content that existed prior to degassing, and the degassing of the melt of steel is continued after at least a portion of the addition agent is added and until the oxygen content of the degassed and recarburized steel is less than 100 parts per million.

10. The method of claim 1 wherein the initial carbon content of the steel is about 0.06-0.08% and the oxygen content is about 500-700 parts per million, the steel is subjected to vacuum degassing until the oxygen content is not greater than about 200 parts per million, and the additionagent is added in an amount to provide the said carbon and exothermic substance in a combined amount of about 0.5-4 pounds per ton of steel, the-residual exothermicsubstance content of the deoxidized and recarburized steel being less than 0.05%.

1 1. The method of claim 1 wherein the'steel is subjected to vacuum degassing until the oxygen content is not greater than about 200 parts per million and the oxygen content in the vacuum degassed and recurburized steel is less than 100 parts per million.

12. The method of claim 1 wherein the addition weight is in the form of briquettes having a size of about 0.25-3

inches.

13. The method of claim 1 wherein the briquettes are in the form of spheres having a size of about 1-3 inches.

14. The method of claim 1 wherein said heat of un: killed steel is produced by a basic oxygen process.

15. The method of claim 14 wherein the steel as pro,- duced by the basic oxygen process contains about 0.06- 0.08% of carbon, the carbon content is lowered to about 0.02-0.04% and the oxygen content to not greater than 200 parts per million during the vacuum degassing step, the addition agent is added in an amount to recarburize the steel and provide a final carbon content of. about 0.06- 0.08%, and the oxygen content in the vacuum degassed and recarburized steel is less than 100 parts per million.

16. The method of claim 15 wherein the addition agent is added in an amount to provide the said carbon and exothermic substance in a combined amount of about 2.0-2.5 pounds per ton of steel, and the residual exothermic substance content of the resulting deoxidized and recarburized steel is less than 0.05% Y References Cited UNITED STATES PATENTS OTHER REFERENCES R-H Degassing, G. B. Forster, Journal of Metals, May 1966, p. 632.

Vacuum Pouring of Ingots for Heavy Forgings, by J. H; Stoll, Journal of the Iron & Steel Institute, January 1959, p. 70. Y

J. SPENCER OVERHOLSER, Primary Examiner V. K. RISING, Assistant Examiner US. Cl. X.R. 

